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Codex is the coding AI agent developed by OpenAI, capable of reading source code on its own, writing features, fixing bugs, running tests in an isolated environment, and then proposing changes for human approval. Unlike a chatbot that only answers questions, Codex works like a “virtual programmer” handling multiple tasks in parallel.
Key points
- A cloud-based software engineering agent, launched in 5/2025, powered by the codex-1 model (fine-tuned from o3) for real-world coding.
- Each task runs in its own container/sandbox: it reads, edits, runs tests, and reports transparent logs, then opens a pull request for review.
- Three ways to use it: Codex in ChatGPT, Codex CLI (open source in the terminal), and GitHub integration.
- Unlike Copilot (suggestions as you type) and regular ChatGPT (conversation): Codex takes on one big task and completes multiple steps.
- Lesson for SMEs/marketers: automation is cheaper and easier, and the skills of clearly describing requirements and thinking in terms of “AI supervision” are becoming more important.
What is Codex? Codex is the coding AI agent developed by OpenAI, capable of reading source code, writing new features, fixing bugs, running tests, and proposing changes to your software project. Unlike a chatbot that only answers questions, Codex works like a “virtual programmer” handling multiple tasks in parallel in a safe, isolated environment. For store owners, SMEs, or marketers just starting to explore technology, understanding Codex helps you see the power of the AI agent wave that is reshaping how businesses build digital products.
What is Codex and why does it matter?
OpenAI Codex (the 2025 version) is a cloud-based software engineering agent, launched by OpenAI during its research phase in May 2025. Codex is powered by codex-1 — a version of the o3 reasoning model fine-tuned specifically for real-world coding work. You can assign Codex multiple tasks at once: add features, answer questions about the codebase, fix bugs, or create a pull request for you to review.
It is important to distinguish this from the name “Codex,” which OpenAI previously used for an older coding AI model released in 2021 (the original foundation of GitHub Copilot), but that model is no longer supported. When people talk about Codex today, they mean the new-generation coding AI agent — a major step beyond simply suggesting individual lines of code. To understand the broader picture of the company behind this technology, you can also read What is OpenAI.
How does Codex work?
The core reason Codex is considered a true “agent” is its ability to carry out multiple steps on its own, rather than just generate text. A typical workflow includes:
- Loading the codebase: Codex runs in a cloud sandbox environment, preloaded with your repository — usually connected through GitHub.
- One container per task: each request is handled in an isolated environment, so tasks do not interfere with one another and the original project is safer.
- Read, edit, and test automatically: the agent reads the project structure, edits files, and runs test commands to verify the result.
- Transparent reporting: Codex provides logs and test results for you to review, then proposes changes or opens a pull request for your approval.
This operating model is very close to the concept of What is an AI agent — an AI system that can plan and act toward a goal on its own, rather than merely responding passively.
Codex CLI and the ways to use Codex
OpenAI offers Codex in several forms to suit different needs:
| Form | Features | Best for |
|---|---|---|
| Codex in ChatGPT | Built directly into the ChatGPT interface, assign tasks via the sidebar, track progress visually | Teams already using ChatGPT and wanting to delegate tasks quickly |
| Codex CLI | An open-source tool that runs directly in the terminal on your machine | Developers who prefer working from the command line |
| GitHub integration | Connects directly to the code repository and opens pull requests for human review | Development teams with a standard review process |
Codex was made available to users of paid ChatGPT plans (such as Plus, Pro, Team, Enterprise) during rollout; access details and limits may change over time, so you should check directly in the official OpenAI documentation or the OpenAI Help Center for the latest information.
How is Codex different from Copilot, Claude Code, and regular ChatGPT?
Many newcomers easily confuse AI coding tools. Quick distinction:
- Regular ChatGPT: a versatile conversational assistant that answers questions and suggests code, but does not autonomously carry out an entire technical task from start to finish.
- GitHub Copilot: mainly suggests and completes code as you type in the editor (smart autocomplete).
- Codex: an agent that can take on a large task, work through multiple steps in a sandbox, and return a complete result for review.
- Similar tools from competitors: Anthropic also has its own coding agent — see What is Claude Code to compare philosophy and usage.
In short: Copilot helps you type faster, while Codex does the whole job for you. This is the core difference between a “typing assistant” and a “coding AI agent.”
What can SMEs and marketers learn from Codex?
You do not need to be a programmer to benefit from this wave. For store owners and marketers, Codex offers a few practical lessons:
- Automation is becoming cheaper and easier: tasks that once required hiring a developer (small website fixes, writing data-processing scripts) can now be supported by agents, helping small teams do more.
- Clearly describing requirements is a golden skill: just like delegating work to a real person, Codex’s output depends on how you express the goal. This skill is similar to writing effective ChatGPT prompts.
- A mindset of “supervising AI” instead of “doing it manually”: the human role shifts to assigning tasks, checking, and approving — a workplace culture change worth preparing for early.
If you want to go deeper into how to apply AI to business work, you can explore our AI Guide section.
Things to keep in mind when starting with Codex
Codex is powerful, but it is not a “magic wand.” A few points to remember:
- Always have a human review the result before deploying to production — the agent can make mistakes or misunderstand the request.
- Protect source code and sensitive data: only connect repositories once you clearly understand the access permissions.
- Start with small, clearly testable tasks, then expand from there.
Official and updated product information is always published on the openai.com homepage and in the product introduction at OpenAI’s Codex launch page.
Frequently asked questions
What is Codex in the shortest possible way?
Codex is OpenAI’s coding AI agent that can write features, fix bugs, and run tests in an isolated environment, then propose changes for human approval.
Can people who do not know how to code use Codex?
Codex is aimed at programmers and software development teams. However, non-specialists should still understand the concept to keep up with the trend and know how to collaborate more effectively with technical teams or technology partners.
Is Codex free?
Codex is being rolled out to users of paid ChatGPT plans, while Codex CLI is an open-source tool. Access and pricing may change, so please check OpenAI’s official documentation and Help Center.
In short, understanding what Codex is helps you grasp one of the most important AI trends today: the shift from a response-based assistant to an AI agent that can act on its own. Whether you are a store owner, SME, or a marketer just starting out, getting familiar early with the mindset of delegating tasks to agents will be a real competitive advantage in the next few years.
